Mean Nocturnal Baseline Impedance (MNBI) Provides Evidence for Standardized Management Algorithms of Nonacid Gastroesophageal Reflux-Induced Chronic Cough

Background The clinical management of nonacid gastroesophageal reflux-induced chronic cough (GERC) is challenging, and patient response to standard antireflux therapy (omeprazole 20 mg twice daily plus mosapride 10 mg thrice daily) is suboptimal. This study aimed to identify predictors of standard antireflux therapy efficacy and provide evidence for standardized management algorithms of nonacid GERC. Methods A total of 115 nonacid GERC patients who underwent multichannel intraluminal impedance-pH monitoring (MII-pH) were enrolled between March 2017 and March 2021. Retrospective analysis of general information and MII-pH indications were used to establish a regression analysis model for multiple factors affecting standard antireflux therapy efficacy. Results 90 patients met the inclusion criteria, and the overall response rate to standard antireflux therapy was 55.5% (50/90). The mean nocturnal baseline impedance (MNBI) (1817.75 ± 259.26 vs. 2369.93 ± 326.35, P = 0.030) and proximal MNBI (1833.39 ± 92.16 vs. 2742.57 ± 204.64, P ≤ 0.001) of responders were lower than those of nonresponders. Weakly acid reflux (56.00 (31.70, 86.00) vs. 14.00 (14.00, 44.20), P = 0.022), nonacid reflux (61.35 (15.90.86.50) vs. 21.60 (0.00, 52.50), P = 0.008), and proximal extent (19.00 (5.04, 24.00) vs. 5.50 (2.56, 11.13), P = 0.011) were markedly higher in responders than nonresponders. Proximal MNBI (OR = 0.997, P = 0.042, and optimal cutoff = 2140 Ω) and weakly acid reflux (OR = 1.051, P = 0.029, and optimal cutoff = 45) were independent predictors of standard antireflux therapy efficacy. The combination predictive value did not show better results than either individual predictor. Conclusions Proximal MNBI < 2140 Ω may be used to screen patients with nonacid GERC suitable for standard antireflux therapy and in standardized management algorithms for nonacid GERC. In the absence of MNBI, weakly acid reflux > 45 can be used as an auxiliary indicator.


Introduction
Gastroesophageal refux-induced chronic cough (GERC), a specifc type of gastroesophageal refux (GER) disease, is one of the most common causes of chronic cough [1,2]. Based on its pH value, GER can be divided into the following two major subtypes: acid and nonacid. Tese subtypes can be further divided into weakly acid and weakly alkaline refux. Refux with pH values ≤ 4, 4.1-7, and ≥ 7 is considered acid, weakly acid, and weakly alkaline refux, respectively [3]. Acid refux is the main cause of GERC, which has led to a considerable amount of research into its diagnosis and treatment. However, nonacid refux, which also plays an important role in GERC, lacks sufcient investigation [4]. Te rate of nonacid GERC has been shown to be 37% and 80% in chronic cough patients without proton pump inhibitor (PPI) treatment and those with PPI treatment, respectively. 26% of chronic cough patients have a positive symptom index (SI), which indicates a nonacid-related cough [5]. Te lack of multichannel intraluminal impedance-pH monitoring (MII-pH) has led to an underestimation of the diagnostic rate and accuracy of nonacid GERC.
Te current frst-line treatment for GERC is PPIs [6,7]. For patients with acid exposure identifed by MII-pH monitoring, 8 weeks of standard antirefux therapy, such as 20 mg omeprazole twice daily plus 10 mg mosapride thrice daily, is recommended. PPIs have been shown to be efective in GERC patients with defnite acid exposure [6,8,9]. Terefore, PPIs have achieved great success in treating GERC, especially acid GERC, by reducing the acidity of regurgitates. Although PPIs are not generally recommended for patients with nonacid GERC [8], studies have shown that nonacid refux is reduced or disappears in some patients receiving PPI therapy. Several previous studies have shown that standard antirefux therapy is efective in more than one-third of nonacid GERC patients [10][11][12][13]. For patients unresponsive to standard antirefux therapy, transient lower oesophageal sphincter relaxations (TLESRs) and oesophageal dysmotility might be the underlying aetiology. Baclofen has shown its antitussive efcacy in these patients by decreasing both acid and weakly acid refux as well as the number of refux episodes and refux-related symptoms [3,11,14]. However, due to the prominent neurological side efects such as sedation, the application of baclofen was usually prudent. Patients had experienced at least 8 weeks of nonresponse to standard antirefux therapy before baclofen initiation, resulting in poor adherence in nonacid GERC cases.
Terefore, this study aimed to identify the potential factors related to the therapeutic efcacy of standard antirefux therapy in nonacid GERC patients. Retrospective analysis of the clinical data of nonacid GERC patients was used to identify nonacid GERC patients for whom standard antirefux therapy was suitable. For patients for whom the therapy was not suitable, baclofen may be administered without unnecessary delay to improve the efciency and accuracy of treatment.

Patients.
Patients with nonacid GERC as the sole explanation of chronic cough attending our respiratory clinic between March 2017 and March 2021 were recruited. After detailed medical history inquiries and physical examination as described previously [1], all patients underwent chest Xray, lung function test, histamine bronchial provocation test [15], capsaicin cough sensitivity test [16], induced sputum cytological examination [17], and MII-pH [11] to identify the causes of chronic cough.
Patients who responded to standard antirefux therapy were kept on this treatment until their cough resolved; otherwise, baclofen was introduced (omeprazole 20 mg twice daily and baclofen initially at 10 mg thrice daily and gradually increased to 20 mg thrice daily) for additional 4 weeks. Nonacid GERC patients who responded to standard antirefux treatment were classifed as the standard antirefuxresponsive group; conversely, the other patients were classifed as the standard antirefux-unresponsive group.
Cough severity was evaluated at the frst visit and every 2 weeks during the follow-up period using the cough symptom score described by Hsu et al. [22]. Te gastroesophageal refux diagnostic questionnaire (GerdQ), described by Xu et al., was employed to evaluate patients' upper gastrointestinal symptoms [12]. Capsaicin cough sensitivity was evaluated according to the European Respiratory Society (ERS) guidelines [8] and using the modifed method described by Fujimura et al. [16] with the lowest capsaicin inhalation concentration required to induce ≥2 (C2) or ≥5 (C5) coughs as the subject's cough threshold.
Patients who met all of the nonacid GERC criteria mentioned above were included in the study. Patients with other causes of chronic cough or incomplete medical records were excluded.
Te study was approved by the Ethics Committee of Tongji Hospital (K-2020-025).

Laboratory Examination.
Measurement of MII-pH [11,23] was carried out by transnasal insertion of a 2.1 mm diameter combined MII-pH catheter consisting of six impedance channel amplifers (K6011-E10632, Unisensor, Switzerland) and an antimony pH probe (819100, Medical Measurement System B.V., Netherlands) into the oesophagus. Te MII-pH catheter was positioned with the pH electrode 5 cm above the lower oesophageal sphincter (LES) and the six impedance recording channels (z1, z2, z3, z4, z5, and z6) at 17, 15, 9, 7, 5, and 3 cm above the LES, respectively. A connected portable data logger (Ohmega; Medical Measurement System BV) was used to store data at 50 Hz frequency from all seven channels over 24 h. MII-pH measures changes in conductivity of an alternating electrical current with a pair of metallic rings mounted on a catheter. Electrical impedance was expressed in ohms (Ω) and was equivalent to the resistance to the direct electrical current. Baseline impedance (BI) [24] values refected the status of the oesophageal mucosa. Event markers were used to record the occurrence of symptoms, times of meals, and changes in posture. Supporting professional analysis software was used for data analysis.
Before MII-pH monitoring, patients who had recently used acid suppressants, potassium chloride, or nonsteroidal anti-infammatory drugs that may cause refux were excluded. During the MII-pH monitoring period, patients were encouraged to perform their regular activities and have routine meals; they were given a personal diary to note the time of meal, sleep, recumbent position, and any coughrelated symptoms. Te following key indicators were recorded using the accompanying professional software (MMS database, v8.7) for analysis: ① acid exposure time (AET) defned as the total time (%) with pH <4 divided by the total time of monitoring; notably, the Lyon Consensus proposes that an AET < 4% is considered normal (physiological) and an AET > 6% is clinically abnormal, with intermediate values between these limits being inconclusive [20]; ② total refux times, where > 80 refux episodes per 24 hours were considered abnormal [20]; ③ the number of proximal refux episodes (15 cm above the LES); ④ the oesophageal acid clearance time (the time required for pH to recover to ≥ 4.0 after acid refux); ⑤ the symptom association probability (SAP) defned as the temporal association between cough recorded by patients in a diary and refux that had occurred during the preceding 2 min period; ⑥ the symptom index (SI) defned as the percentage of refuxrelated symptom episodes (cough) among the total number of symptom episodes; ⑦ the mean nocturnal baseline impedance (MNBI) [25] assessed from the most distal impedance channel during the nighttime recumbent period; three 10 min time periods (approximately 1.00 am, 2.00 am, and 3.00 am) were selected, and the mean was calculated, without periods of swallows and refuxes or pH drops; distal MNBI was calculated as the average of MNBI values from the channels located at 3, 5, 7, and 9 cm above the LES; proximal MNBI was calculated as the average of MNBI values from the channels located at 15 and 17 cm above the LES; and ⑧ postrefux swallow-induced peristaltic wave (PSPW) index [25], where PSPWs were defned as antegrade 50% drops in impedance, originating in the proximal oesophagus and reaching the distal lumen within 30 s after refux events. Te PSPW index was calculated by dividing the number of PSPWs by the number of refux events. Te MNBI and PSPW index were calculated by the same operator blinded to the treatment outcomes of each patient.

Review of Clinical Information.
A retrospective analysis was performed to compare the general information, MII-pH monitoring parameters, and other assessments between the responsive and nonresponsive groups. Stepwise logistic regression was then performed to identify the indicators of the initial assessments of the therapeutic response to standard antirefux therapy in nonacid GERC patients.

Statistical Analysis.
Te normally distributed data are expressed as the mean ± standard deviation (SD), while skewed data are expressed as the median (interquartile range). After logarithmic transformation, C2 and C5 were expressed as the geometric mean ± SD. Gender and coughing properties were compared by the χ 2 test. Te analysis of variance and the Kruskal-Wallis rank sum test were used to compare the data between groups when applicable. Spearman and Pearson's tests were used for the correlation analysis. A univariate regression analysis was performed to screen signifcant variables, and then, a stepwise multiple logistic regression analysis was used to identify independent predictors for the therapeutic efcacy of standard antirefux therapy. Receiver operating characteristic (ROC) curves were plotted for the model equation and then compared using the DeLong test. SPSS 21.0 software (SPSS, Chicago, IL, USA) was applied for statistical calculation. A P value <0.05 was considered statistically signifcant.

General Information.
A total of 975 patients with complete information over the period studied were diagnosed with chronic cough. Of these, 273 were diagnosed with GERC. A total of 115 nonacid GERC patients met the inclusion criteria of this study, accounting for 42.1% of GERC patients in the same period. Four patients with incomplete follow-up data and twenty-one patients with dual/ triple aetiologies were excluded. Of the 21 excluded patients, 6 had nonacid GERC combined with cough variant asthma (CVA), 5 had upper airway cough syndrome (UACS), 5 had eosinophilic bronchitis (EB), and 1 had obstructive sleep apnoea syndrome (OSAS). In addition, 4 patients had 3 aetiologies, including 2 patients with nonacid GERC plus UACS and CVA, 1 patient with nonacid GERC plus UACS and OSAS, and 1 patient with nonacid GERC plus UACS and EB. Te remaining 90 out of the original 975 patients were included in the study, as they had only one aetiology (nonacid GERC).

Diferences in MII-pH Data of Responders and
Te other indices, including AET, SAP, SI, and total refux times, which are important indicators for the diagnosis of GERC, in addition to the PSPW index, were not signifcantly diferent between the two groups ( Table 2).

Factors Associated with the Terapeutic Efcacy of
Standard Antirefux Terapy. Te variables with statistically signifcant diferences between the responders and nonresponders in Tables 1 and 2 were selected for the univariate logistic analysis. Among the signifcant factors identifed by univariate logistic analysis, multivariate logistic regression revealed that proximal MNBI (OR � 0.997, P � 0.042) and weak acid refux (OR � 1.051, P � 0.029) were independent predictors of standard antirefux therapy (Table 3).
When P ≥ 0.3772 was used as a cutof point, the logistic regression equation efectively discriminated the responders from the nonresponders, with a sensitivity of 87.5% and a specifcity of 83.3%, as indicated by the ROC analysis ( Figure 1).

Proximal MNBI and Weak Acid Refux for Predicting
Standard Antirefux Terapy Efcacy. When using the proximal MNBI to predict the efcacy of standard antirefux therapy for nonacid GERC, the AUC ROC was 0.789 (P � 0.001), and the optimal cutof value for proximal MNBI was 2140 Ω (Figure 2). Using a proximal MNBI of < 2140 Ω as the predictive indicator for therapeutic efcacy, the AUC ROC was 0.814 (P � 0.001), with a Youden index of 0.629, positive predictive value of 68.42%, negative predictive value of 93.33%, specifcity of 70%, and high sensitivity of 92.86% (Table 4). Using weak acid refux as a predictor, the AUC ROC was 0.761 (P ≤ 0.001), with an optimal cutof of 45 ( Figure 2). When weak acid refux > 45 was used as a predictor, the AUC ROC was 0.686 (P � 0.001) with a Youden index of 0.373, specifcity of 62.96%, sensitivity of 74.29%, positive predictive value of 65.20%, and negative predictive value of 72.22% (Table 4).

Discussion
In this study, nonacid GERC patients who responded to standard antirefux treatment had a lower MNBI and proximal MNBI and more episodes of weakly acid refux, nonacid refux, gas refux, and proximal extent refux than nonresponders. Multivariate logistic regression revealed that  International Journal of Clinical Practice proximal MNBI and weakly acid refux were independent predictors of standard antirefux therapy. In addition, further analysis combined with the ROC curve analysis showed that the diagnostic value of proximal MNBI < 2140 Ω was higher than that of weak acid refux > 45.
Nonacid refux was characterized by the pH value and further separated into weakly acidic (pH 4-7) or weakly alkaline (pH>7) [3]. While most studies have focused on acid GERC, nonacid GERC has also been reported and can sometimes be the crucial aetiology [27][28][29][30]. A meta-analysis    Figure 1: Te internal accuracy of the logistic regression model assessed by ROC curve analysis. Te AUC was 0.924 (95% CI: 0.749-0.9591; P ≤ 0.001), which indicates that for 92.4% of the paired participants (one responder, one nonresponder), the responder scored higher. Tese results suggest that the logistic regression model used in this study had a good ability to discriminate between responsive and unresponsive participants. AUC, area under the curve; CI, confdence interval; ROC, receiver operating characteristic. [28] revealed that in SAP-positive GERC patients in whom acid suppressive therapy was discontinued for 2 weeks, the percentages of acid, weakly acidic, and weakly alkaline refux were 65%, 29%, and 6%, respectively. In another study [5], 37% of the patients with GERC in whom PPIs had been withdrawn had nonacid GERC. However, in patients undergoing antiacid therapy, nonacid refux occurred at a higher rate (80%-82.7%) [5,31]. We previously [29] demonstrated that weakly acid refux was the main cause of cough symptoms in nonacid GERC [29]. Refux episodes reaching the proximal oesophagus and the pharynx were observed in nonacid GERC patients [32,33]. Among them, the incidence of nonacid refux was fairly high, accounting for 73% in the proximal oesophagus and 11.1% in the pharynx [32].
In recent years, nonacid GERC has been increasingly diagnosed with the use of MII-pH monitoring. However, the treatment of nonacid GERC is still clinically challenging. Neither the chronic cough guidelines [2,7,8,21] nor the recent Lyons Consensus [20] illustrate the efective management of nonacid GERC. Both types of GERC (acid and nonacid) patients were pretreated with standard-dose PPIs for 8 weeks when there was defnite abnormal refux [7,34]. However, some nonacid GERC patients who did not respond to standard antirefux therapy [8,34] did respond to subsequent neuromodulators, such as baclofen [3,10,11,13,14]. For this reason, it is necessary to identify nonacid GERC patients who respond to the standard antirefux treatment early and directly treat these patients with neuromodulators, which can quickly alleviate their cough symptoms, improve their compliance, and avoid unnecessary treatment, thus improving the overall efcacy of nonacid GERC.
Tis study revealed that proximal MNBI and weakly acid refux were independent predictors of standard antirefux therapy. During MII-pH monitoring, diferent media (gas, liquid, and mixture) generated diferent impedances when passing through the electrode patches at each end of the monitoring catheter, and the MNBI was a stable value when the subjects were at a steady state during the night without a bolus passing through. Prolonged exposure of the oesophageal epithelium to luminal acid increased the permeability of the oesophageal epithelium in a dose-dependent manner, in part by modulating the amount of claudin-1 and claudin-4 [35]. Tis resulted in decreased transepithelial resistance, increased paracellular permeability, and dilated intercellular spaces (DIS) [36]. Tus, the baseline impedance (MNBI) decreased [36]. Acid exposure correlated with macroscopic and microscopic lesions in oesophageal mucosa. Te persistence of weakly acid refux containing bile  acids and/or other gastric juice components may induce or maintain both DIS and oesophageal permeability [37]. Distal oesophageal perfusions provoked changes not only in the "exposed" but also in the more proximal "nonexposed" mucosa. A recent study by Caviglia et al. [38] showed that the DIS in NERD patients may lie in both areas close to the gastroesophageal junction and the more proximal oesophagus, which is less exposed to gastric refuxate. Tis "spread of DIS from the distal to proximal oesophagus" may be related to mast cell degranulation (histamine release), activation of capsaicin-sensitive aferent neurons, and release of neurokinins, which are involved in oesophageal hypersensitivity. Te human oesophageal mucosa is very sensitive to continuous exposure to acidic solutions. Oesophageal perfusion with weakly acidic solutions induced DIS of similar magnitude to that provoked by acidic solutions [39]. Terefore, weakly acidic and acidic solutions are equally capable of causing MNBI reduction. Previous studies in our department have demonstrated that proximal refux is mainly caused by weakly acidic refux [40]. Clinical studies have found that 75% of nonacid refux reached the upper third of the oesophagus [41], which is consistent with our fndings in this study. Te responders had more weak acid refux and proximal refux than the nonresponders. In addition, studies have shown that the ability of weakly acidic refux to cause DIS and thus to reduce MNBI is comparable to that of acid refux [39]. In this study, more weakly acidic refux reached the proximal oesophagus and caused direct damage to the proximal oesophageal mucosa, resulting in a decrease in the proximal MNBI. Furthermore, we demonstrated that weak acid refux could be transmitted from the lower oesophagus to the high "nonexposed zone" oesophagus via the aforementioned oesophageal hypersensitivity refex [38], which in turn causes a further decrease in the proximal MNBI. We suggest that this is the reason why the responders had a lower proximal MNBI than the nonresponders and that the predicted antirefux efect was due to the superposition of the direct stimulation of the proximal weakly acidic refux and the stimulation of distal refux through the oesophageal hypersensitive refex. In this study, nonacid GERC patients with weakly acid refux > 45 and proximal MNBI < 2140 Ω still had increased acid exposure (weakly acid); thus, treatment with a standard dose of PPIs alone was sufcient to reduce the acidity of the refux content.
A signifcant proportion of patients in this study responded to standard antirefux therapy. Combined with previous studies in our department [11][12][13]18], the reason why standard antirefux works in some patients with nonacid GERC can be attributed to some of the following reasons: (1) Patients with nonacid GERC were rigorously screened, and other possible causes of cough were excluded according to a step-by-step procedure. Combined with the objective parameters of MII-pH, the diagnosis of nonacid GERC was fnally established. (2) Patients were amenable to PPIs and were diferent from those refractory to a high dose of PPIs reported in previous studies [3]. (3) Te role of small amounts of acid refux cannot be ruled out even though nonacid refux is the main cause of cough (this can be seen in the results of this study, which showed that patients who responded to standard antirefux had more weak acid refuxes). (4) Te slight increase in the pH value of the refuxate caused by PPIs may efectively reduce the stimulation of weakly acid refux to oesophageal mucosa [3]. (5) Prokinetic drugs promote the emptying of the oesophagus and stomach and reduce the volume of refuxate and frequency of refux. Oesophageal dysfunction and airway infammation and hypersensitivity are more likely to be the underlying aetiologies in patients with nonacid GERC who do not respond to standard antirefux treatment. Te additional treatment of a muscle relaxant such as baclofen can not only reduce both types of refux (acid and nonacid) by inhibiting transient lower oesophageal sphincter relaxation but also act as a nonspecifc antitussive medication [11].
Although logistic regression showed that both proximal MNBI < 2140 Ω and number of weak acid refux > 45 could predict the therapeutic efect of standard antirefux therapy, ROC curve analysis indicated a higher predictive value of proximal MNBI. Te relatively low sensitivity (62.96%) of weakly acid refux > 45 as a therapeutic efcacy predictor might cause the majority of patients with nonacid GERC to be omitted. In addition, given the high susceptibility of weakly acid refux events to be afected by the patient's lifestyle (i.e., the type of food, drugs, and meal), the proximal MNBI demonstrates its relatively stable and reliable characteristics as an objective indicator. Terefore, in future, a proximal MNBI < 2140 Ω can be used to accurately predict the response to standard antirefux therapy in nonacid GERC patients. In addition, weakly acid refux > 45 can also be used as a suboptimal secondary predictor when MNBI data are not available. Tese two indicators can be used to target the treatment of nonacid GERC, quickly relieving the cough symptoms of some nonacid GERC patients and achieving an overall improvement in treatment efciency and accuracy. Notably, a recent study [42] in which suspected GERC patients were stratifed according to AET (>6%; between 4% and 6%; and < 4%; erosive oesophagitis was excluded) reported a correlation between pathological MNBI or PSPW index and PPI response. Te diferences between the results of our study and those mentioned above can be explained by the diferent study subjects. However, both studies propose that MNBI, a novel MII-pH variable, can make up for the limits of AET and SAP/SI.
Te major limitations of this report are the small sample size and recruitment from a single centre. Further verifcation with a prospective study is needed. However, given that nonacid refux is not that common in the GERC population, it is difcult to achieve a large sample size. Placebo efects can also not be excluded. However, it is not a clinical drug trial but a retrospective observational study, and remarkable improvement has been reported previously [10][11][12]14].
Collectively, standard antirefux therapy should be applied directly to nonacid GERC patients with a proximal MNBI < 2140 Ω. In the absence of MNBI, weakly acid refux > 45 can be used as an auxiliary indicator. Baclofen is recommended as an additional treatment after standard antirefux therapy.

Data Availability
Te datasets used and/or analysed during the current study are available from the corresponding authors upon request.

Ethical Approval
Te study procedure was approved by the Ethics Committee of Tongji Hospital (no. K-2020-025).

Consent
Written informed consent was obtained from all participants.

Conflicts of Interest
Te authors declare that they have no conficts of interest.